Cooling of a gas turbine Stator Heat Shield (SHS), particularly of first stage, is a very challenging task. Indeed, film cooling of hot gas exposed surface actively used for blading components is hardly applicable to the area where the rotating blade passes the SHS for two reasons. First, the complex flow field in the gap between SHS and blade tip does not allow for cooling film development and the resulting film effectiveness is low and hard to predict. Second, in case of rubbing events, cooling holes openings can be closed, thus preventing required cooling air outflow, which would have a detrimental effect on the whole cooling system and reduced lifetime.
As a result, very common practice for state-of-art SHS cooling is to use extensive impingement cooling with cooling air discharged from side faces of SHS through convective holes, which limits overall cooling effectiveness.
Further development of heavy duty gas turbine engines (e.g. for combined cycle) is focused on the raise of cyclic parameters: pressure ratio and hot gas temperature. In long-term perspective hot gas path components will be obliged to survive turbine inlet hot gas temperature of 2000-2200 K and available convective cooling schemes will not be feasible to guarantee proper lifetime of first stage SHS's even despite of noticeable increase of discharge areas and air-to-hot-gas pressure ratio.
The second potential issue caused by an excessive growth of turbine inlet temperature is the worsening of lifetime of blade tip region that is typically exposed by the most severe thermal conditions driven by geometrical restrictions and high turbulence level in the tip clearance region. To increase the lifetime in this specific area to an acceptable level it would require noticeable increase of cooling flow rates by opening discharge areas. This action would have a detrimental impact on overall turbine and engine efficiency. Moreover it should be stressed high discreteness between hot gas and coolant flows in the blade tip region and any local hot gas streak can cause a life-limiting location.
The majority of known cooling schemes for stator heat shields deals with mature manufacturing technologies (casting, machining, brazing) and conventional cooling features (impingement, pins and cylindrical holes).
The wider spread scheme is a combination of impingement with side discharge, as disclosed for instance in US 2012/0251295 A1 and U.S. Pat. No. 6,139,257. All these schemes are robust but due to the limitations within only convective cooling with discharge through long holes in front, side and rear of the SHS limits their cooling efficiency within the state-of-art level.
US2005/0058534 A1, U.S. Pat. No. 5,538,393 propose serpentine cooling schemes and EP2549063 A1 proposes helix shaped cooling scheme. Although the given cooling schemes are quite effective due to high heat utilization rates, again their cooling efficiency is limited by fixed coolant to hot gas pressure head and absence of any kind of external cooling. Special words should be said about low adjustability of design towards nonuniform external boundary conditions.
US2009/0035125 A1, U.S. Pat. Nos. 5,165,847, 5,169,287, 6,139,257, 6,354,795 B1 and EP 1533478 A2 propose impingement cooled SHS with cooling air ejection at hot has exposed surface. This schemes allow to maximize pressure head and impingement heat transfer rates and convective cooling efficiency of the components, however all those disclosures are suffering from the following: in case of rubbing event, risk of which always exists in heavy duty gas turbines, cooling hole exits can be closed thus preventing cooling airflow and consequently cause overheating of the SHS. Moreover due to positioning of discharge holes towards trailing edge of the blade, cooling of the blade tip is not considered in the aforementioned teachings.
US 2012/0027576 A1 and US 2012/0251295 A1 propose effusion cooling scheme revealing cooling air at the complete hot gas washed surface of SHS. Again, no mitigation against rubbing is given, and the part is critical for the installation in case of tight radial clearances.
WO2013129530A1 proposes an example of external “film” cooling organization within deep retaining grooves; however no cooling proposals to cool down thick metal area between the grooves were given.